In recent years, a display device (or a display module) that is provided with a touch sensor as a position-input means has been put to practical use. A display device (or a display module) that is provided with a touch sensor is called a touch panel, a touch screen, or the like (hereinafter, this type of display device may be simply referred to as a touch panel). A device which does not include a display device and includes only a touch sensor is also called a touch panel in some cases. A display device that is provided with a touch sensor is also called a touch sensor equipped display device, a display device equipped touch panel, a display module, or the like in some cases. Furthermore, a display device in which a touch sensor is incorporated is called an in-cell touch sensor (or an in-cell touch sensor equipped display device), an on-cell touch sensor (or an on-cell touch sensor equipped display device), or the like in some cases. In the in-cell touch sensor, for example, an electrode used for a liquid crystal element is also used as an electrode for the touch sensor. In the on-cell touch sensor, for example, an electrode for the touch sensor is formed on the upper side (the side that is not provided with a display element) of a counter substrate. Examples of a portable information terminal provided with such a touch panel or the like include a smartphone and a tablet terminal.
As one of display devices, there is a liquid crystal display device provided with a liquid crystal element. For example, an active matrix liquid crystal display device in which pixel electrodes are arranged in a matrix and transistors are used as switching elements connected to respective pixel electrodes has attracted attention.
For example, active matrix liquid crystal display devices in which a transistor formed using a metal oxide for a channel formation region is used as each of switching elements connected to respective pixel electrodes are known (see Patent Documents 1 and 2).
Touch panels in which a liquid crystal element is used are disclosed in Patent Documents 3 to 6.
It is known that liquid crystal display devices are roughly divided into two kinds of liquid crystal display devices: a transmissive liquid crystal display device and a reflective liquid crystal display device.
In the transmissive liquid crystal display device, a backlight such as a cold cathode fluorescent lamp or an LED is used, and optical modulation action of liquid crystal is utilized to select one of the two states: a state where light from the backlight passes through liquid crystal to be output to the outside of the liquid crystal display device and a state where light is not output to the outside of the liquid crystal display device, whereby a bright or dark image is displayed. Furthermore, those images are combined to perform image display.
In the reflective liquid crystal display device, optical modulation action of liquid crystal is utilized to select one of the two states: a state where external light, that is, incident light is reflected on a pixel electrode to be output to the outside of the device and a state where incident light is not output to the outside of the device, whereby a bright or dark image is displayed. Furthermore, those images are combined to perform image display.